In apparatus for combustion, oscillations frequently take place in the combustion chamber with frequencies within the subsonic or audible ranges, which result in the disturbing of the environment by noise and may lead to damage to the plant. These oscillations are, however, undesirable also from an energy standpoint.
There are involved self-excited acoustic oscillations of the gases in the combustion chamber and in the pipelines which feed the combustion air as well as the fuel gas. In practice, these detrimental oscillations are generally controlled purely empirically by more or less systematic changes in the geometry of the burner or by variation of the composition of the fuel gas. This generally leads to unsatisfactory results and practically always to compromises in which undesired disadvantages must be tolerated.
In German Pat. No. 23 50 338 there is disclosed a device for the avoidance of combustion-chamber oscillations. Within the gas feed is incorporated a hollow space as a reflection chamber which has a spacing of one-quarter acoustic wavelength from the burner mouth at that natural frequency of the combustion chamber at which self-excitation is to be expected or feared. This device has proven itself excellent in many blast heater installations which are originally equipped with these reflection chambers. The reflection chambers provide the desired result. Even in case of strong variation of the composition of the fuel gas, no oscillations were observed. However, with this device it is necessary that the critical natural frequency be precisely calculated in advance before the plant is constructed, which sometimes is difficult. In addition to this, the device is to be arranged outside the combustion chamber, which means that additional space is required. The subsequent incorporation of such reflection chamber frequently encounters considerable difficulties, at least in the case of large installations, since structural space is then necessary, for the creating of which other parts of the plant frequently have to be shifted to another place.